CORE Metadata, citation and similar papers at core.ac.uk Provided by RERO DOC Digital Library Swiss J Geosci (2011) 104:1–29 DOI 10.1007/s00015-010-0049-6 Timing of Palaeozoic magmatism in the Maggia and Sambuco nappes and paleogeographic implications (Central Lepontine Alps) Denise Bussien • Franc¸ois Bussy • Tomas Magna • Henri Masson Received: 30 November 2009 / Accepted: 1 November 2010 / Published online: 16 February 2011 Ó Swiss Geological Society 2011 Abstract Magmatic rocks from the pre-Mesozoic base- affiliation of the Sambuco basement. The calc-alkaline ments of the Sambuco and Maggia nappes have been dated by Matorello pluton yielded ages around 300 Ma, similar to U–Pb zircon ages with the LA-ICPMS technique. Several numerous Late Carboniferous intrusions in other basement magmatic events have been identified in the Sambuco nappe. units of the Lower Penninic (Monte Leone, Antigorio, The mafic banded calc-alkaline suite of Scheggia is dated at Verampio) and Helvetic domains (Gotthard and other Exter- 540 Ma, an age comparable to that of mafic rocks in the nal Crystalline Massifs). Associated lamprophyric dykes are Austroalpine Silvretta nappe. The Sasso Nero peraluminous slightly younger (300–290 Ma), like similar dykes sampled in augengneiss has an age of 480–470 Ma, like many other gneiss blocks included in the sedimentary cover of the ‘‘older orthogneisses’’ in Alpine basement units. It hosts a underlying Antigorio nappe (290–285 Ma). The Cocco large proportion of inherited zircons, which were dated around granodiorite and Ru¨scada leucogranite, both intruding the 630 Ma, a Panafrican age indicating the Gondwanan basement of the neighbouring Maggia nappe, yielded ages of ca. 300–310 Ma, identical within errors to the age of the Matorello pluton. They are significantly older than former age Editorial handling: Edwin Gnos. determinations. This age coincidence, coupled with remark- able petrologic similarities between the Cocco and Matorello D. Bussien Á H. Masson granodiorites, strongly suggests paleogeographic proximity of Institute of Geology and Paleontology, the Sambuco and Maggia nappes in Late Carboniferous times. University of Lausanne, Anthropole, 1015 Lausanne, Switzerland In recent publications these two nappes have been interpreted as belonging to distinct Mesozoic paleogeographic domains: Present Address: ‘‘European’’ for Sambuco and ‘‘Brianc¸onnais’’ for Maggia, & D. Bussien ( ) separated by the ‘‘Valais’’ oceanic basin. In this case, the Geologisches Institut, ETH Zu¨rich, 8092 Zu¨rich, Switzerland similarity of the Matorello and Cocco intrusions would e-mail: [email protected] demonstrate the absence of any significant transcurrent movement between these two continental domains. Alterna- F. Bussy Á T. Magna tively, according to a more traditional view, Sambuco and Institute of Mineralogy and Geochemistry, University of Lausanne, Anthropole, Maggia might belong to a single large Alpine tectonic unit. 1015 Lausanne, Switzerland Keywords LA-ICPMS U–Pb dating Á Zircon Á T. Magna Granite Á Variscan Á Penninic Á Swiss Alps Institut fu¨r Mineralogie, Universita¨tMu¨nster, 48149 Mu¨nster, Germany 1 Introduction: Paleozoic orogenies in the Central Alps Present Address: T. Magna Czech Geological Survey, Kla´rov 1, Pre-Mesozoic geology of the Alps is recorded in the base- 118 21 Prague 1, Czech Republic ments of the nappe stack that forms the belt. Although the 2 D. Bussien et al. basement history is still relatively poorly known if compared to Variscan magmatic events tend to become younger the Mesozoic and Tertiary record, modern research based on toward the internal parts of the Alpine belt (Bussy and isotopic dating and geochemistry of magmatic rocks has Cadoppi 1996). The External Massifs do not record intru- thrown a new light on the ancient evolution of the Alpine sions younger than 295 Ma (the Rotondo granite in the basement. Two major events have been documented: a Cam- Gotthard massif; Sergeev et al. 1995), and the Lower brian–Ordovician cycle (sometimes abusively called Penninic nappes, which are their southern extension, Caledonian), and a Variscan or Hercynian cycle of Middle younger than ca. 290 Ma (Verampio and Antigorio; Devonian to Permian age. Clues of older events have been Bergomi et al. 2007). Further south, the Brianc¸onnais identified in a few places, like in the Silvretta nappe (Austro- domain contains very few Variscan intrusions older than alpine), where a metadiorite has been dated at 609 ± 3Maand 270 Ma and only in its distant continuation in Italy: the a gabbro-tonalite calc-alkaline series at ca. 525 Ma, interpreted Costa Citrin granite at 323 ± 8 Ma (Zone Houille`re Bri- as evidence for an island-arc setting (Schaltegger et al. 1997). anc¸onnaise; Bertrand et al. 1998) and the Borda The Cambrian–Ordovician orogenic cycle starts with ca. granodiorite at 300–294 Ma (Ligurian Brianc¸onnais; 500 Ma old A-type alkaline granites attributed to conti- Gaggero et al. 2004). All other Variscan intrusions in the nental rifting, recorded in the Brianc¸onnais domain (Guillot Brianc¸onnais basement have ages B270 Ma. et al. 1991; Bussy et al. 1996) and in the Silvretta nappe The present study focuses on the dating and character- (Austroalpine domain; Mu¨ller et al. 1995). A complete ization of Palaeozoic magmatic rocks from the basements of collisional sequence is documented in the Aar massif, with the Sambuco and Maggia nappes in the Central Lepontine early HP metamorphism affecting a gabbro protolith Alps. Both nappes host large and various magmatic bodies, formed at 478 ± 5 Ma, Barrovian metamorphism and partly dated by Ko¨ppel et al. (1981) using large, discordant decompression partial melting at ca. 450 Ma (Schaltegger zircon fractions which yielded suspiciously young ages in et al. 2003). Bodies of peraluminous and calc-alkaline or- the light of recent data on Variscan granites. Here we will thogneisses of similar age (480–450 Ma) are ubiquitous in date these rocks by an in situ method (the LA-ICPMS the basements of all paleogeographic domains, attesting the technique) that provides U–Pb ages of small individual wide extension of this orogeny (e.g. Bussy and von Raumer domains in single zircon crystals. We also investigated 1994; Poller et al. 1997; Schaltegger and Gebauer 1999; blocks of gneiss with lamprophyre dykes from a meta- Bertrand et al. 2000; Guillot et al. 2002). wildflysch formation in the sedimentary cover (Teggiolo Variscan relics are even more widespread and particularly zone) of the underlying Antigorio nappe, in order to con- well preserved in the External Crystalline Massifs of the strain their paleogeographic provenance. The results show Alpine belt (e.g. Aar, Gotthard, Aiguilles-Rouges and Mont- that the Sambuco nappe records all the main Palaeozoic Blanc). Early and still undated mafic eclogites are preserved magmatic episodes known in other basement units of the in the Lake Cornu area (Aiguilles-Rouges; Lie´geois and Alps. In addition, granites from the Maggia nappe are Duchesne 1981; von Raumer and Bussy 2004). Nappe significantly older than formerly proposed by Ko¨ppel stacking led to Barrovian metamorphism (U–Pb monazite et al. (1981), which casts some doubts about recent ages of 327 ± 2 Ma in the Aiguilles-Rouges, Bussy et al. paleogeographic models of the Central Alps (see 2000; and 317.5 ± 2 Ma in the Aar, Schaltegger et al. 2003) below). and partial melting (U–Pb monazite age of 321 Ma in the Aiguilles-Rouges; Bussy et al. 2000) in a fast exhuming environment (Capuzzo and Bussy 2000; Genier et al. 2008). 2 Geological framework and open controversies Syn- to post-orogenic magma pulses of dominantly granitic composition record different stages of the evolving orogen at The Maggia and Sambuco nappes (Fig. 1) are situated in ca. 330, 310 and 300 Ma (Schaltegger 1997; Bussy et al. the Lepontine Alps, where insight into the deepest nappes 2000; von Raumer and Bussy 2004). of the Alpine stack is allowed by the Ticino structural This general picture stands for the External Crystalline culmination. Most of this region is covered by the tectonic- Massifs, which form the basement of the Helvetic realm of petrographic map (1: 100,000) of the central Lepontine the Alps, i.e. the relatively stable part of the European margin Alps (Berger and Mercolli 2006) and the western sector by of the Alpine Tethys. Things are less clear in more internal the tectonic map (1:100,000) of the Western Swiss Alps zones, as precise ages are scarcer and field relationships are (Steck et al. 1999). In the sectors that concern us directly obscured by Alpine tectonics, which dissected the basement these synthetic maps are essentially based on the sheets Val into multiple nappes. Conversely, Palaeozoic geology can Bedretto (Hafner et al. 1975), Basodino (Burckhardt and provide helpful constraints in the reconstruction of the Gu¨nthert 1957) and P. Campo Tencia (Keller et al. 1980)of Mesozoic paleogeographic puzzle, as will be attempted in the 1:25,000 Geological Atlas of Switzerland where many this study. relevant details are figured. Timing of Palaeozoic magmatism 3 Fig. 1 Geological map of the N Basel northern Maggia area, with 690 Bern Zürich sample localities (black stars) 150 150 and sample names (white Lausanne DB19 boxes). Modified from Berger and Mercolli (2006), Simpson 0 km 100 Laghetti DB66 Lago P. Massari (1981) and Steck et al. (1999). Naret Coordinates of the swiss 680 DB77 Scheggia DB38 Lago Sambuco topographic grid DB61 DB76 DB23 DB25 Cristallina DB26 Robiei P. di Röd DB29 Lebendun